1. Field of the Invention
The present invention relates to an apparatus for removing unnecessary data in communication networks, which is used in a television transmitting and receiving devices of a cable television system and the like. In particular, the present invention relates to an apparatus for removing unnecessary data in communication networks, which, in a television data transmitting and receiving devices for carrying out data communication, undesired and unnecessary data such as a command for change of an address loaded in a data stream transmitted from a transmitting end to a receiving end are detected, and the cyclic redundancy check is changed, so that the receiving device can ignore the data stream, thereby removing the unnecessary data.
2. Description of the Prior Art
Transmitting and receiving devices for a conventional cable television broadcasting system are illustrated in FIG. 1. Referring to FIG. 1, a transmitting device 100 of a broadcasting station transmits data streams in block units. Then the transmitted data streams are sent through a network amplifier 150 to a cable television receiving device 200.
A data stream which is transmitted from the transmitting device of a broadcasting station is constituted as shown in FIG. 2. That is, its format is: 1 byte of flag, 2 bytes of address, 1 byte of command, 16 bytes of data, and 2 bytes of cyclic redundancy check (CRC). That is, the data streams are transmitted in such a sequence in block units.
In the case where the CRC of the transmitting data stream is normal, the cable television receiving device 200 processes the received data in the normal manner. On the other hand, in the case where a CRC error has occurred, the received data are ignored.
In this conventional system, in the case where a command which is not wanted by the cable television receiving device 200 is present in the data stream which are transmitted by the transmitting device 100 of the broadcasting station, that is, in the case where an address change command or the like is received to the receiving device 200, the internal programs and system of the cable television receiving device 200 cannot operate in the normal manner. Not only so, but a serious damage can be inflicted to the expensive software or hardware.
FIG. 3 is a block diagram showing the constitution of another conventional address data correcting apparatus which is disclosed in Japanese Patent Publication No Hei-4-63579.
Referring to FIG. 3, this address data correcting apparatus operates in the following manner. That is, a data transmitting device transmits data having address data which have a certain number of differences between the continuous address data of the digital data which are transmitted in block units. For these transmitted data, the receiving end synchronizes a plurality of the received address data. Then differences between the synchronized continuous address data are detected. Then the detected differences and the mentioned certain number are compared to form block signals which display the presence or absence of errors. In the case of presence of an error, of the correct address data indicated by a flag among the plurality of the address data, there is calculated the time lag value on the address data which is closest to the erroneous data in time, thereby correcting the error.
In this conventional address data correcting apparatus, data errors are detected and corrected without carrying out the encoding for detecting and correcting errors. However, there is no feature of checking unnecessary data for the command data. Therefore, even if an address is duly corrected, if an address change command is transmitted, the system address of the receiving end as well as the transmitting address is changed, resulting in that a fatal damage is inflicted to the hardware or software